added track of theme changes

Display icons for open file tabs

fix https://github.com/ethereum/remix-project/issues/849

Display icons based on file types.

This addresses an issue when you rename a folder which has files which are open in tabs.
It would recreate the current opened file because it is current and saved when another file opened. You'd get duplicate files. Also tabs from the old folder would still be visible.
Now the behavior should be: tabs that belong to the folder renamed will be closed and replaced. No file will be focused.
Please test.

Select files on hover and context menu display

Fix saving of empty content

fixes two bugs:
- removing spaces from ids in remix-tabs breaks finding the tab again through this._handlers[name]
- duplicate tabs are re-created twice.

fix https://github.com/ethereum/remix-project/issues/793

Improve annotations